1. Field of the Invention
The present invention relates to an image forming apparatus which can form images on both sides of a sheet, and the apparatus includes, for example, a copying machine, a printer, and a facsimile machine.
2. Description of the Related Art
Conventionally, some image forming apparatuses including a copying machine, a printer, a facsimile machine, and the like have a mode in which images are formed on both sides of a sheet (hereinafter, it will be called a double-sided image forming mode).
When images are formed on both sides of a sheet by such double-sided image forming, the sheet is turned over by a sheet turning over portion provided in the image forming apparatus, using a switch back method. FIG. 10 shows an explanatory view of a side section of a known sheet turning over portion. Operations for turning over processing of a sheet will be described referring FIG. 10.
When a sheet is conveyed from a conveying path 101 to a conveying roller 102, a switching member 108 provided in a branch portion of the conveying roller 102 is rotated to open a conveying path 104 and to close the side of a conveying guide 103. Thereby, the sheet is conveyed to the conveying path 104 by the rotation of the conveying roller 102.
As a switching member 109 provided in the branch portion of a turning over roller 105 closes a conveying guide 110 when the sheet is conveyed to the conveying path 104, the sheet passes the conveying path 104, and is sent off to a sheet turning path 107 by the turning over roller 105. The turning over roller 105 stops when the rear end of the sheet passes the switching member 109.
When the turning over roller 105 stops, the switching member 109 closes the conveying path 104 to open the conveying path 110. The turning over roller 105 sends off the sheet to the conveying guide 110 by reversing the direction of rotation, and the sheet is conveyed as it is, by a conveying roller 106 to the image forming portion through a both-sided conveying path.
In the conventional sheet turning over apparatus using a switch back method as described above, a roller for switching back a sheet has to be stopped and reversed when a sheet is turned over, which wastes time.
Moreover, when a sheet is switched back, the sheet is temporarily taken into a sheet turning path 107 and the apparatus is occupied with the sheet until the whole sheet passes the guide 110. Thereby, longer paper requires more time to be taken in. Accordingly, it has been difficult to improve the productivity in the double-sided image forming mode.
There has been proposed a method by which a sheet is twisted and turned, as shown in FIG. 11. As shown in FIG. 11, a turning over unit 30 has a structure in which an attracting belt 33 is stretched between a belt drive roller 35, which is a convey starting position, and a belt drive roller 36, which is a convey completing position, while being twisted 180 degrees. The sheet is attracted onto the attracting belt 33 by adding electrostatic force to the attracting belt 33 using an AC power supply 41, and the sheet is conveyed. Thereby, the time required for turning over the sheet is shortened to improve the productivity at the double-sided image forming mode (refer to Japanese Patent Application Laid-Open No. 2006-103855).
However, the method for twistingly turning over requires a space in which a sheet is rotated by 180 degrees and is turned over. Accordingly, the size of an image forming apparatus becomes larger.
Moreover, a sheet conveying speed in an image forming apparatus has been increased in recent years. As a result, behaviors at the end of a sheet become unstable at delivery of a sheet from a roller conveying portion to the subsequent roller conveying portion, and at delivery of a sheet from a roller conveying portion to a belt conveying portion in conventional roller conveying, since roller conveying and belt conveying exist in a complicated manner in a conventional apparatus. As a result, sheet jamming (clogging) occurs which results in unstable sheet conveying.